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Mon, 17 May 2021 01:34:25 -0700 (PDT) MIME-Version: 1.0 References: <20210507114048.138933-1-omosnace@redhat.com> <24a61ff1-e415-adf8-17e8-d212364d4b97@schaufler-ca.com> <94486043-322f-74bd-dc33-83e43b531068@schaufler-ca.com> In-Reply-To: <94486043-322f-74bd-dc33-83e43b531068@schaufler-ca.com> From: Ondrej Mosnacek Date: Mon, 17 May 2021 10:34:12 +0200 Message-ID: Subject: Re: [PATCH] lockdown,selinux: fix bogus SELinux lockdown permission checks To: Casey Schaufler Cc: Linux Security Module list , James Morris , Steven Rostedt , Ingo Molnar , Stephen Smalley , SElinux list , linuxppc-dev@lists.ozlabs.org, Linux FS Devel , bpf , network dev , Linux kernel mailing list Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=omosnace@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: On Sat, May 15, 2021 at 2:57 AM Casey Schaufler wrote: > On 5/14/2021 8:12 AM, Ondrej Mosnacek wrote: > > On Wed, May 12, 2021 at 7:12 PM Casey Schaufler wrote: > >> On 5/12/2021 9:44 AM, Ondrej Mosnacek wrote: > >>> On Wed, May 12, 2021 at 6:18 PM Casey Schaufler wrote: > >>>> On 5/12/2021 6:21 AM, Ondrej Mosnacek wrote: > >>>>> On Sat, May 8, 2021 at 12:17 AM Casey Schaufler wrote: > >>>>>> On 5/7/2021 4:40 AM, Ondrej Mosnacek wrote: > >>>>>>> Commit 59438b46471a ("security,lockdown,selinux: implement SELinux > >>>>>>> lockdown") added an implementation of the locked_down LSM hook to > >>>>>>> SELinux, with the aim to restrict which domains are allowed to perform > >>>>>>> operations that would breach lockdown. > >>>>>>> > >>>>>>> However, in several places the security_locked_down() hook is called in > >>>>>>> situations where the current task isn't doing any action that would > >>>>>>> directly breach lockdown, leading to SELinux checks that are basically > >>>>>>> bogus. > >>>>>>> > >>>>>>> Since in most of these situations converting the callers such that > >>>>>>> security_locked_down() is called in a context where the current task > >>>>>>> would be meaningful for SELinux is impossible or very non-trivial (and > >>>>>>> could lead to TOCTOU issues for the classic Lockdown LSM > >>>>>>> implementation), fix this by adding a separate hook > >>>>>>> security_locked_down_globally() > >>>>>> This is a poor solution to the stated problem. Rather than adding > >>>>>> a new hook you should add the task as a parameter to the existing hook > >>>>>> and let the security modules do as they will based on its value. > >>>>>> If the caller does not have an appropriate task it should pass NULL. > >>>>>> The lockdown LSM can ignore the task value and SELinux can make its > >>>>>> own decision based on the task value passed. > >>>>> The problem with that approach is that all callers would then need to > >>>>> be updated and I intended to keep the patch small as I'd like it to go > >>>>> to stable kernels as well. > >>>>> > >>>>> But it does seem to be a better long-term solution - would it work for > >>>>> you (and whichever maintainer would be taking the patch(es)) if I just > >>>>> added another patch that refactors it to use the task parameter? > >>>> I can't figure out what you're suggesting. Are you saying that you > >>>> want to add a new hook *and* add the task parameter? > >>> No, just to keep this patch as-is (and let it go to stable in this > >>> form) and post another (non-stable) patch on top of it that undoes the > >>> new hook and re-implements the fix using your suggestion. (Yeah, it'll > >>> look weird, but I'm not sure how better to handle such situation - I'm > >>> open to doing it whatever different way the maintainers prefer.) > >> James gets to make the call on this one. If it was my call I would > >> tell you to make the task parameter change and accept the backport > >> pain. I think that as a security developer community we spend way too > >> much time and effort trying to avoid being noticed in source trees. > > Hm... actually, what about this attached patch? It switches to a > > single hook with a cred argument (I figured cred makes more sense than > > task_struct, since the rest of task_struct should be irrelevant for > > the LSM, anyway...) right from the start and keeps the original > > security_locked_down() function only as a simple wrapper around the > > main hook. > > > > At that point I think converting the other callers to call > > security_cred_locked_down() directly isn't really worth it, since the > > resulting calls would just be more verbose without much benefit. So > > I'm tempted to just leave the security_locked_down() helper as is, so > > that the more common pattern can be still achieved with a simpler > > call. > > > > What do you think? > > It's still a bit kludgy, but a big improvement over the previous version. > I wouldn't object to this approach. Ok, thanks. I'll post it as a v2 then. -- Ondrej Mosnacek Software Engineer, Linux Security - SELinux kernel Red Hat, Inc.